Symptoms of abnormal heart rhythms are generally referred to as cardiac arrhythmias, with an abnormally rapid heart beat being referred to as tachycardia. The present invention is concerned with the treatment of tachycardias that are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the surface of the upper chambers of a heart. The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper chamber to the lower chamber necessary for performing normal systole and diastole functions. The presence of arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
Treatment of tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. Catheter based radiofrequency ablation is generally performed after conducting an initial electrophysiology study where the locations of the arrhythmogenic site and/or accessory pathway are determined. Electrophysiology study and temporary defibrillator/cardioverter involve catheters having at least one electrode means for the delivery of low energy pacing pulses directly to the heart tissue at various intervals or frequencies in order to induce the arrhythmia. Once the arrhythmia is induced, it must be terminated by typically delivering a high-energy defibrillation shock across the patient's chest with paddles. A specific very high therapeutic voltage is required within the heart to terminate the induced arrhythmias. The voltage delivered through a conventional transthoracic defibrillator paddle from outside the body is substantially greater than the necessary therapeutic voltage because it needs to compensate for the energy losses through the skin and the thoracic cavity. An internal defibrillator/cardioverter from a catheter poses less risk and higher therapeutic efficiency.
A transvenous catheter that combines the functions of the rate sensing, pacing, and defibrillation has a plurality of electrodes on a distal portion. The catheter should have high torque so that it can be guided to the exact location of the heart tissue for pacing, cardioversion, defibrillation and/or radiofrequency ablation. The catheter should also be flexible enough for insertion into a body and have adequate surface for contacting the intracardiac tissues. Some examples of cardioverter and methods are U.S. Pat. Nos. 5,005,587 to Scott, 5,766,224 to Alferness et al., 5,814,081 to Ayers et al., 5,810,887 to Accorti, Jr. et al., and 5,913,887 to Michel. Because of the exact location requirements, the torque transmission properties of a catheter to ensure that a substantial fraction of the rotational and displacement forces provided along the catheter are translated to rotational and displacement forces at the distal end of said catheter become very important.
Another particular interest point to the present invention is radiofrequency (RF) ablation protocols that have proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablation tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy is then applied through the electrode to the cardiac tissue in order to ablate the tissue of arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal conducted signal patterns responsible for the tachycardia can be eliminated. In the case of atrial fibrillation (AFib), multiple arrhythmogenic sites and/or multiple accessory pathways exist. It becomes desirable that a long linear lesion or a broad lesion zone at the tissue contact sites to ensure appropriate energy delivery.
Scott in U.S. Pat. No. 5,005,587 discloses a braided electrode lead in an endocardial pacing and defibrillation catheter having an elongated hollow polyurethane shaft. Although providing good stiffness and biocompatibility, the polyurethane tube lacks sufficient translation between the rotational and displacement forces applied along the body of the polyurethane tube. Accorti Jr. et al. in U.S. Pat. No. 5,810,887 discloses a catheter having a braided wire section for torque transmission. For the purposes of cardioversion and defibrillation, the surface contact area becomes very critical. With regards to the braided wire section, only the outer radial surface of the braided section is employed in contacting a tissue. In other words, the effective tissue-contacting surface of wires in the braided wire section is sub-optimal as compared to an essentially flat surface. The braiding of wires also makes the braided wire section undesirably rigid.
Similarly, Michel in U.S. Pat. No. 5,913,887 discloses a transvenous device having three coil electrodes for cardioversion of atrial flutter or atrial fibrillation. The tissue-contacting surface area of a coil electrode is less than an essentially flat surface. The coil electrode also suffers a drawback of less torque transmission property. None of the conventional temporary catheters for cardioversion/defibrillation has a steerability property.
While cardioversion and defibrillation procedures using an existing catheter design have produced some promising results, the torque transmission properties, the effective tissue-contacting surfaces, and rigidity of a conventional catheter usually do not meet the clinical requirements. Therefore, there is an urgent clinical need to have a flexible catheter that also possess high torque transmitting property to be used in cardioversion and defibrillation procedures.